The number of pixels in each line of a display such as a LCD is usually fixed, while that designed for a horizontal line of a video card varies with different products, resulting in the possibility that the number of pixels in a video signal exceeds what a display (such as a LCD) can display line bar line.
A display usually displays messy images as a result that the active part of a video signal falls beyond the right boundary of its displaying region, leading to the need that an user does manual adjustment according to his experience and luck, to restore the order of displaying the video signal. Even the user eventually succeeds in the adjustment, the displayed messy images and the associated adjustments make people frustrated and annoyed.
Therefore it is expected by many people and related industries that a circuit or a process is designed to provide a solution of maintaining stable image displaying on the screen of a display such as a LCD.
When displaying a video signal in a display based on a sequence of synchronous pulses, the active part of the video signal must be between two successive synchronous pulses, in order to display the image of a video signal in a proper region line by line on the screen of the display. As shown in FIG. 1. active part 11 of the video signal must be between two synchronous pulses 13 or within a synchronous cycle 15 (a cyclical period), to assure the image 17 carried by the video signal is displayed in a region line by line corresponding to the sequence of synchronous pulses, i.e., line by line in a region determined according to the continuous synchronous pulses such as the synchronous pulses 13.
Sometimes it may happen that the active part 11 of a video signal extends endlessly, as shown in FIG. 2a, as a result of synchronization failure, leading to a permanent mess on the screen of the display.
A block diagram showing a conventional algorithm for adjusting the timing of the active part of the video signal for displaying image line by line is shown in FIG. 2b, where process 32 inputs a video signal 41, a signal 45 representing the setting or adjusting by users, and synchronous pulse 46, for detecting the synchronization of image and adjusting the timing of starting image displaying. Without design of relevant detection process or circuit, adjusting for the synchronization of image relies solely on users' experience and luck instead of automatic operation. It can thus be seen that the conventional algorithm does not monitor closely the timing of the active part of a video signal, thereby can't prevent synchronous failure from causing serious screen mess.